The present invention generally relates to an image forming apparatus for forming image information by executing image drawing commands, and more particularly relates to an image forming apparatus capable of effectively describing and reproducing a drawn graphics image using a gradation pattern whose colors are progressively changed, and a method of generating a gradation pattern.
Heretofore, as document composition systems and document print systems using computer systems are sophisticated, representation techniques used therein are also sophisticated. In order to execute the effective presentation and the solid geometric representation, there has been used a technique called gradation for progressively changing a color of a certain graphics image area.
While a document composition software is able to designate gradation by using a variety of user interfaces, this imposes a large load on the printer side when the printer prints the gradation. The reason for this is that a page description language for describing document data is unable to effectively describe the gradation. As a result, to realize the gradation, a page description language is described by the procedure in which regions of the same shape but having slightly different colors are drawn in an overlapping fashion with each other with slight displacements. For example, in a gradation in which colors are changed smoothly in a concentric circle fashion, it is necessary to repeatedly execute the processing in which a large circle is drawn and filled with a pattern and a little smaller circle is drawn on the above circle with a different color in an overlapping fashion and then filled with a pattern until the position reaches the center position of the circle. Thus, when the page description language processing system generates the gradation pattern, it is unavoidable that the complicated procedure should be executed, thereby resulting in the processing load being increased.
On the other hand, Japanese Published Unexamined Patent Application No. Hei 8-72317 describes a method of realizing a gradation on the printer side. This method introduces a gradation description as a part of description method of a page description language. When this page description is interpreted, the gradation pattern expanded as the background is cut out by the shape of the drawing target graphic form, thereby resulting in the gradation within the printer being realized. Also, according to this method, in order to draw a gradation of a circumference shape, a boundary of very small concentric circle-shaped regions is calculated by using a DDA (digital differential algorithm). DDA is a well-known algorithm for computing a position at which an intersection point of a graphic form crossing a certain scanning line is moved on the next scanning line. By using this algorithm, all intersection points with the scanning line are computed previously and the filling processing between the intersection points is executed.
However, in accordance with the method described in Japanese Published Unexamined Patent Application No. Hei 8-72317, since the gradation pattern larger than the graphic form shape is one expanded and cut out along the graphic form shape, it is necessary to compute colors of other regions than the drawing target region. Moreover, in the case of the gradation of the concentric circle shape, the proceeding direction of the computation is directed in the boundary of the concentric circle-shape region, which processing is not suitable for the filling processing on the scanning line. There is then the problem that independent processing steps are required.